 IBM's New Attachment Strategy beginning with the 3830 Model 2 in 1972 physically separated the SCU into two physical entities, a director and a controller while keeping them logically the same.The controller handles the CKD track formatting and is packaged with the first drive(s) in a string of drives and having a model number with the letter "A" as a prefix, an "A-Unit" (or "A-Box") as in 3350 Model A2 containing a controller and two DASDs.DASD without a controller, that is B-Units, have a "B" prefix in their model number.